This invention relates generally to a liquid crystal display (LCD), and more particularly to a high contrast LCD microdisplay utilizing compensated high-speed column video switches.
For many decades, the cathode ray tube (CRT) was the dominant display device creating an image by scanning a beam of electrons across a phosphor-coated screen causing the phosphors to emit visible light. The beam is generated by an electron gun and is passed through a deflection system that causes the beam to rapidly scan left-to-right and top-to-bottom. A magnetic lens focuses the beam to create a small moving dot on the phosphor screen. This rapidly moving spot of light paints an image on the surface of the viewing screen.
Light emitting diodes (LEDs) have also found a multitude of uses in the field of optoelectronics. An LED is a solid-state device capable of converting a flow of electrons into light. By combining two types of semiconductive material, LEDs emit light when electricity is passed through them. Displays comprised of LEDs may be used to display a number of digits each having seven segments. Each segment consists of a group of LEDs, which in combination can form alphanumeric images. They are commonly used in, for example, digital watch displays, pager displays, cellular handset displays, etc., and due to their excellent brightness, LEDs are often used in outdoor signs. Generally speaking, however, they have been used primarily in connection with non-graphic, low-information-content alphanumeric displays. In addition, in a low-power CMOS digital system, the dissipation of LEDs or other comparable display technology can dominate the total system""s power requirements, which could substantially negate the low-power dissipation advantage of CMOS technology.
Liquid crystal displays (LCDs) were developed in the 1970s in response to the inherent limitations in the then existing display technologies (e.g. CRTs, LED displays, etc.) such as excessive size, limited useful life, excessive power consumption, and limited information content. LCD displays comprise a matrix of pixels that are arranged in rows and columns that can be selectively energized to form letters or pictures in black and white or in a wide range of color combinations. An LCD modifies light that passes through it or is reflected from it as opposed to emitting light, as does an LED. An LCD generally comprises a layer of liquid crystalline material suspended between two glass plates or between a glass plate and a substrate. A principle advantage of an LCD over other display technologies is the ability to include thousands or even millions of pixels in a single display paving the way for much greater information content.
With the shift from segmented, very low information content displays to more information-rich digital products, LCDs now appear in products throughout the communications, office automation, and industrial, medical, and commercial electronics industries. Historically, the market for small displays has demanded low cost, minimal power consumption, and high image quality. To assure high quality, high contrast images, it is necessary that the voltage stored on the pixel capacitors match, as closely as possible, the original source video signal voltage. Thus, the video signal must not be distorted when the source video signal coupled to each of the individual columns of pixels is switched on and off. For example, in a CMOS microdisplay, distortion of the video may be caused by what is commonly referred to as charge injection and clock feed-through occurring when an NMOS access transistor column-switch enable signal is turned off.
In view of the foregoing, it should be appreciated that it would be desirable to provide a high-contrast, high-quality LCD microdisplay wherein the video image stored on the pixel capacitors matches the original source video signal. This is accomplished by utilizing a high-speed CMOS column video switches that include compensating transistors for injecting opposite charge onto the video line when the column video switch turns off. Additional desirable features will become apparent to one skilled in the art from the foregoing background of the invention and the following detailed description of a preferred exemplary embodiment and appended claims.
In accordance with the teachings of the present invention, there is provided an LCD display for generating an image of a video signal that includes a matrix of pixels arranged in a plurality of rows and a plurality of columns, which are selectively energized to create the image. The rows are connected to a row select circuit for energizing each of the rows in accordance with a first predetermined sequence. The columns are coupled to a column select circuit that couples the video signal to each of the columns in accordance with the second predetermined sequence. The column select circuit includes a plurality of video switches, each of which include a high speed current mirror level shifter for shifting the control signal from a first potential to a second higher potential, and a transmission gate for coupling the video signal to one of the columns upon receipt of the higher potential control signal.